Most of the media properties listed on this chart are what could be called “one-way media”: Professionals produce a media product that is broadcast on TV, published in print or posted on the web, and an audience consumes it. Done. End of story.

Three companies not on this chart are revolutionizing the way millions of Americans inform and entertain themselves and one another: Google, Yahoo! and Microsoft. These companies–none of which set out to be media companies–are now innovators in new forms of participatory media, born of the read-write web. Microsoft runs the world’s biggest blogging platform, MSN Spaces. Google transformed the way most of us get our information with a search engine that enables us to find citizen-created media content alongside the work of professionals. It now also owns several services, like Blogger.com, that enable anybody with minimal technical knowledge or equipment to create and disseminate media. Yahoo!, which started out as a search engine, is now a full-blown media company enabling users to customize their own mix-and-match diet of news, information, opinion and entertainment produced by a range of professional sources as well as their friends, colleagues and amateur strangers. Yahoo! also provides easy-to-use services enabling people to publish, share and disseminate their own media creations with relative ease and little cost.

In this dawning era of blogging, MySpace (the online community site wildly popular with teenagers, now owned by News Corporation) and YouTube (a year-old video-sharing startup likely to be bought up soon by one of the big players), the latest Pew study has found that 48 million American adults–35 percent of all adult Internet users–have created original content and posted it on the web. And that doesn’t count all the kids on MySpace. Internet entrepreneur Robert Young recently wrote that over the past year we have witnessed the birth of a new media sector: the self-expression sector, taking its place alongside TV, radio, magazines, newspapers and the largely one-way websites run by many of the companies on this chart that allow very limited, if any, user participation.

It now looks pretty certain that, before long, Google, Yahoo! and Microsoft will have displaced some of the companies on the chart and acquired or merged with others. None of the one-way media companies are driving innovation in self-expression media, though some have gotten into the game thanks to purchases of or partnerships with other innovators (and a bit of cautious copycatting). News Corporation, with MySpace, has gone furthest in its embrace of the web’s full read-write potential. For some reason Time Warner’s AOL has done little to innovate in this arena–but will its new partnership with Google change that? Or maybe Google should just buy Time Warner?

As the self-expression sector grows into a multimillion-dollar industry, those of us who care about media’s role in democracy need to start thinking about what this means. Certainly, the read-write web empowers people who didn’t have the resources to get their own voices heard in the past, and has the potential to empower many more. But who participates in self-expression media? Who has ready access to cheap broadband connections and computers and who doesn’t? Will the rise of the self-expression media sector mean that people with easy broadband access at home and work will gain in their influence over the American political discourse, while the vast unwired will find it even harder to get their perspectives heard? Will universal access to high-speed Internet–along with widespread web education–become prerequisites for an inclusive democratic discourse in this country?